This invention relates to a method of producing flowing gas mixtures having constant compositions over periods of time, and more particularly to a method of providing a gas mixture stream whose component concentrations are continuously adjusted to maintain the ratio of the components of the gas mixture substantially constant over extended periods of time. The invention is especially useful for accurately filling gas cylinders with gas mixtures of desired compositions.
Gases that are to be shipped to various locations are generally packaged in portable vessels of various shapes and sizes which are capable of withstanding high pressures and which can be conveniently shipped. Typical of such vessels are the cylindrical containers commonly known as gas cylinders or bottles. These vessels are generally filled with gases by charging the gas into the vessel until the desired pressure is reached. The procedure is relatively simple and problem-free when the gas cylinder is to contain a single gas. However, when a gas container is to be filled to high pressure with a gas mixture, it is more difficult to precisely measure the quantities of all of the components of the gas mixture. Filling gas containers with mixtures is particularly problematic when the mixture is desired at high pressures because gases do not obey the ideal gas laws under such conditions, and, in fact, each gas behaves differently at high pressures.
High pressure containerized binary gas mixtures are generally prepared by charging one component into the container until a selected pressure is reached and then charging the second component into the container until the final pressure is reached. The selected pressure is that which corresponds to the partial pressure of the first component in the desired gas mixture. Unfortunately, because of the non-uniform nature of gases at different pressures, it is difficult or impossible to exactly produce the desired gas mixture.
The problem is further complicated when a container is to be filled with a gas mixture comprising a large concentration of one component, for example concentrations of 75 volume % or more, and small quantities of one or more other components, for example concentrations of 10 volume % or less of each minor component. In such cases the inherent inaccuracy of pressure gauges magnifies the error as the desired concentration of a component decreases. A conventional procedure for filling gas cylinders with gas mixtures comprising a minor component and a major component is to first introduce the minor component into the cylinder using a low pressure gauge, and then introduce the major component into the cylinder to the desired end pressure using a high pressure gauge. Since precision pressure gauge readings are usually accurate to within about 0.1% of full scale, the error will be small when this procedure is used. A disadvantage of this method is that different gauges are required for measuring the components of the gas mixture. Furthermore, if the minor compound is heavier than the major component, the first-filled minor component remains separated at the bottom of the gas cylinder for a prolonged period of time.
Methods and systems for accurately filling vessels with gas mixtures have been considerably investigated. U.S. Pat. No. 3,653,414 discloses a system and method for charging a thermostat with a mixture of a condensable medium and a noncondensable gas. The noncondensable gas is first introduced into the sensor of the thermostat to a predetermined pressure, measured by a first pressure gauge. A quantity of the condensable medium, measured by difference in pressure using a second pressure gauge, is then introduced into the sensor.
U.S. Pat. No. 3,669,134 discloses a gas measuring method in which two gases are charged into separate chambers using separate pressure regulators that are interconnected in such a manner that the pressures of the gases are in a predetermined ratio. The apparatus and method disclosed in this patent is complex and difficult to apply, particularly when it is desired to produce mixtures of three or more gases.
U.S. Pat. Nos. 3,856,033 and 3,948,281 disclose a method of filling gas containers with mixtures of gases by continuously mixing the gases at low pressure and then pressurizing the gas mixture and subjecting the high pressure mixture to infrared analysis to determine the concentration of each component in the gas mixture. If the high pressure mixture does not have the desired composition, adjustments are made in the relative rate of flow of the components to the low pressure mixing zone to reduce the variation from the desired composition.
U.S. Pat. No. 4,219,038 discloses a gas mixing device for mixing a plurality of gases wherein each gas flows through a line that has a pressure regulator. In one embodiment of the disclosed invention the individual gases are stored in batteries of containers.
U.S. Pat. No. 4,688,946 discloses a method of mixing a liquid organic compound and a liquid propellant involving filling a metering cylinder with the liquid organic compound and then forcing the liquid organic compound, together with a predetermined volume of liquid propellant, into a mixing vessel.
U.S. Pat. No. 4,698,160 discloses apparatus for mixing two fluids for use in hemodialysis. Syringe type piston pumps are used to measure and force one or more of the components of the mixture into a mixing vessel.
U.S. Pat. No. 5,353,848 discloses procedure for accurately metering the components of a gas mixture into a gas cylinder while avoiding gas stratification, by introducing the gases into the cylinder in the order of their molecular weights using a differential pressure gauge.
U.S. Pat. No. 5,427,160 discloses a method of charging an oxidant gas and a flammable gas into a storage vessel wherein separate measuring chambers are used for each gas. The residual gas in the system lines is vented from the system.
Because of the importance of providing containerized gas mixtures in which the components of the mixtures are in precise composition, and the need to attain immediate homogeneity of vessel-contained gas mixtures, improved gas vessel filling methods are continuously sought. The present invention provides a method and system which accomplishes these objectives.
According to one aspect, the invention comprises a method of producing a gas mixture of substantially constant composition. The method comprising: (a) establishing liquid phase flow of each component to be included in the desired mixture; (b) vaporizing the components; (c) mixing the components; (d) analyzing the vaporized mixture to determine the concentration of each component in the mixture; (e) where necessary, adjusting the phase flow rate of one or more of the liquefied components in a manner that will reduce the difference between the measured composition and the desired composition of the gas mixture; and (f) periodically repeating step (e). The components may be first vaporized and then mixed, or they may be first mixed and then vaporized. Flow of the liquid components is established using a variable output pump means. In a preferred embodiment the flow of liquid is established and maintained using variable speed pumps. In another preferred embodiment, the pressure of the flowing components upstream of the point of mixing is controlled by backflow pressure regulating means.
In another preferred embodiment, the concentration of the components is determined using a gas analyzer. The gas analyzer is preferably an infrared gas analyzer or a mass spectrometer. In a more preferred embodiment, the gas analyzer sends a signal to a control system which adjusts the output of one or more of the pump means.
In one preferred version of the invention, the method additionally comprises filling gas containers, preferably gas cylinders with the constant composition gas mixture. In this version the gas mixture stream may be discarded, if its composition is outside acceptable concentration limits, until its composition is brought to within the desired composition range limits by relative adjustment of the liquid flow rates.
In another preferred version, the method additionally comprises using the constant composition gas mixture as a feed gas to a chemical reaction.
In another aspect of the invention, a system for providing flow of gas mixtures of precisely measured components is presented which comprises as major components:
(a) a flow control means for controlling the rate of flow of two or more vaporizable liquids;
(b) liquid vaporizing means;
(c) fluid mixing means;
(d) gas mixture analyzing means; and
(e) flow adjustment control means.
The system also includes a control loop for transmitting a signal from the gas mixture analyzer means to the flow adjustment control means upon determination of the composition of a gas mixture flowing through the system; a control loop for transmitting flow adjustment signals from the flow adjustment control means to the flow control means; conduit means providing fluid communication between each source of vaporizable liquid and the flow control means; conduit means providing fluid communication between the flow control means and one of either the liquid vaporizing means or the fluid mixing means; conduit means providing fluid communication between the liquid vaporizing means and the fluid mixing means; conduit means providing fluid communication between the other of either the liquid vaporizing means or the fluid mixing means and a downstream application; and a sampling device for transmitting to said gas analyzer means samples of fluid passing through the conduiting between the other of either the liquid vaporizing means or the fluid mixing means and the downstream application.
In a preferred embodiment of this aspect of the invention, the downstream application is a gas container filling system.
In one embodiment of the system aspect of the invention, conduit means provides fluid communication between the flow control means and the liquid vaporizing means and conduit means provides fluid communication between the fluid mixing means and the downstream application. In another embodiment of the system conduit means provides fluid communication between the flow control means and the fluid mixing means and conduit means provides fluid communication between the liquid vaporizing means and the downstream application.
In another preferred embodiment of the invention, the gas mixture analyzer is an infrared analyzer or a mass spectrometer. In another preferred embodiment, the flow control means is a variable delivery liquid pumping means.
In a preferred embodiment of the apparatus aspect of the invention, the system further comprises back pressure control means for controlling, at a point upstream of the fluid mixing means, the pressure of fluid flowing in either (a) the conduit means that provides fluid communication between the flow control means and one of the liquid vaporizing means or the fluid mixing means, or (b) the conduit means that provides fluid communication between the liquid vaporizing means and the fluid mixing means.